When heat-curable resin powder coatings are applied, they do not give off any organic solvents and therefore have clear ecological advantages over liquid coating compositions including paints. The cross-linking of such resins under heat takes place by way of polyaddition or polycondensation reactions between the functional groups contained in the resin binders. Typical binder systems are epoxy resins/curing agents; carboxyl-polyester/epoxies, hydroxyl-polyester/isocyanates, hydroxyl-acrylates/isocyanates, carboxyl-acrylates/epoxies, epoxy-acrylates/dicarboxylic acids, carboxyl-polyester/or carboxyl-acrylates/b-hydroxyalkylamides, and so forth. The binder systems listed differ from one another not only in their technical properties as paints and coating layers, but especially in their resistance to outdoor weathering; but all have high-gloss surfaces, or in other words a gloss index of &gt;80 (DIN 67530, angle of incidence 60.degree.). There is presently an increasing need for coated layers from powder with matte or semigloss surfaces with a gloss index of &lt;60 at an angle of incidence of 60.degree. (DIN 67530) and good outdoor weathering resistance for applications such as architecture, automobiles, and furniture.
The matting additives usual in the liquid paint industry, such as chalks, finely dispersed silicic acids and oxides, talc, etc., exhibit only a very slight matting effect in powdered paints, and when added in relatively large quantities they make the technical properties for painting purposes unacceptably worse. Although additives that are incompatible with the binder, such as waxes, etc., do produce so-called silken-sheen surfaces (gloss index &gt;60 at an angle incidence of 60.degree.), nevertheless in actual use the enrichment of incompatible additives has undesirable effects on the surface.
U.S. Pat. No. 3,842,035 therefore proposes preparing matte powdered paint coatings by what is known as dry-blending of finished powdered paints of adequately differing reactivity, in other words powdered paints with very short and very long gelation times. Although this process leads to coatings of the desired matte quality, nevertheless dry blending of finished powdered paints is very complicated and expensive, and demixing, especially with overspray return, means that a uniform, reproducible surface is no longer assured. There has accordingly been no lack of attempts to produce powder coating surfaces with a matte surface by coextrusion of all the components without demixing problems and with good replicability.
German Patent Disclosure DE-A 2 324 696 proposes a process for preparing matte coatings by using a special curing agent that reacts with epoxy groups, the agent being the salt of cyclic amidines, with certain polycarboxylic acids. Cross-linking of the powdered paint occurs in this process with differing reactivity at different temperatures, and as a result microstructures that have a matte surface form on the coating surface. However, the use of this process is limited to epoxy- and carboxyl-polyester/epoxy powdered paints, with the exception of carboxyl-polyester/triglycidylisocanurate (TGIC) powdered paints, and therefore powdered paints with adequate outdoor weathering resistance cannot be produced by this process.
European Patent Disclosure EP 0 366 608 B1 also proposes a process for producing powdered paints with matte surfaces. It relates to powdered paints based on epoxy resins or epoxy compounds, such as triglycidylisocyanurate (TGIC) and mixtures of di-, tri- or tetrakis-(b-carboxyethyl)cyclohexanones, or di-, tri- or tetrakis-(b-carboxyethyl)cyclopentanones, and carboxyl-terminated polyester resins. The matte effect is ascribed here to the differing reactivity between the aliphatic carboxyl groups of the cross-linking agent and the aromatic carboxyl groups of the carboxyl-terminated polyester resin.
Finally, German Patent Disclosure DE-A 3 232 463 describes the production of coatings with matte surfaces from a powder binder system based on hydroxyl-terminated polyester resins, epoxy compounds such as TGIC, and special reversibly blocked polyisocyanates with free carboxyl groups as cross-linking agents. The blocked polyisocyanates contain more than one blocked isocyanate group per molecule, have an acid value of from 20 to 150 mg KOH/g, preferably 25 to 80 mg KOH/g, and an NCO content/acid value quotient of 0.075 to 0.340, preferably 0.100 to 0.300. This disclosure states it to be especially surprising that the acid value or the NCO content is not solely responsible for the attainment of matte surfaces, but rather the ratio between isocyanate and carboxyl groups, even though under the usual curing conditions for powdered paints, isocyanate groups and carboxyl groups react hardly at all with one another. With a binder system described in another patent reference (EP A 0 056 167), which also has a blocked polyisocyanate cross-linking agent containing carboxyl groups but has a different ratio of isocyanate to carboxyl, only high-gloss coatings are therefore attained.
Although the processes proposed in the patent disclosures EP 0 366 608 B1 and DE A 3 323 463, after coextrusion of all the components, lead to matte surfaces with good outdoor weathering resistance, nevertheless both binder systems have the substantial disadvantage that they contain epoxy compounds, especially triglycidylisocanurate (TGIC), and are therefore extraordinarily objectionable on toxicological grounds.